The present invention relates to an improvement for refractory plates for the side containment in continuous casting machines of thin flat products such as a strip, and to a process for the manufacture thereof.
Usually, continuous casting machines of thin flat bodies have a pair of parallel counterrotating rolls which internally are cooled with water. At each side thereof face two flat walls, each of them having a refractory plate, thus constituting the mould. During casting, the plates are kept abutting onto each side of the rolls, thus enabling the side containment of the molten metal. The molten product cast between said rolls forms a thin flat product.
Various problems exist, due to the high temperatures reached at the starting and in the operation for these types of apparatuses. In fact, reaching high temperatures entails remarkable strains to the refractory plates and the casting rolls. Plate strains are already apparent at the preheating stage, whereas the roll strains derive from the contact of the same with the molten metal during the casting for thin strip formation. These strains, mainly taking place at the initial stages of casting, make it difficult to realize a correct coupling of the refractory plates onto the arc of contact with the casting rolls. This can promote molten metal penetrations, actually hindering the effective containment of the molten metal inside the mould.
Therefore, in order to overcome this problem, the plate material should be relatively soft, thus enabling the rotating rolls to swiftly abrade the plate material at the start of the process to obtain a satisfactory surface coupling, thus providing a seal against molten metal penetrations.
On the other hand, in order to ensure long enough casting times, the plate is required not to wear out too rapidly during the steady state.
Moreover, the adopted material must possess non-wettability and chemical inertia characteristics towards the molten metal, as well as be resistant to the thermal shock caused by the contact with the molten metal at the start of casting. The materials most widely used in the state of the art in forming the portion of the plate contacting the rolls and the molten metal are products made of BN, SiON and/or compounds thereof, as a monolithe or as inserts.
Such types of materials possess the aforedescribed chemical and physical characteristics. Being relatively soft products, these materials have a low abrasion wear and thermal resistance.
The refractory plates for the side containment thus formed are subject to rapid wear, unavoidably leading to rapid consumption of the aforesaid plates, resulting in premature shutdown of the casting process. This problem raises great difficulties in the industrial manufacture of big volumes of thin flat products such as strips or the like.
Over the years, various technical solutions for lubrication of the area of contact between the refractory plates and the rolls have been proposed, aimed at reduction of friction, and therefore the wear of the plate materials. One of these solutions lies in the use of liquid lubricants, e.g. various greases and oils, or solid lubricants, such as graphite and boron nitride that are fed at the entrance of the contacting area between the refractory plates and the edges of the rolls. In the operative thermal conditions, the solvent-carried lubricants exhibit low effectiveness, as in practice they are quickly degraded and volatilized, whereas the solid lubricants can penetrate only a small fraction of the contacting area between the roll and the refractory plate. Moreover, in both instances steel bath pollution problems might arise.
Another technical solution is disclosed in EP 0588743 B1, providing thin film coatings onto the aforesaid plates, made with self-lubricating polymers and having thicknesses of between 0.01÷0.1 mm, added to the refractory materials at the contact area with the rolls.
Such coatings, although charged with glass and/or metallic fibers, are unavoidably destined to rapid wearing due to the scantily added thickness thereof and the thermal conditions of the process, both for the abrasive action exerted by the compression of the rotating rolls and for the operative temperatures of the system. All this makes the use of self-lubricating polymeric structures, even special ones such as TEFLON or PFE, non suitable to overcome the problems of the wear of the refractory plate at the area contacting the rolls, and the correct coupling thereof.
However, self-lubricating films of reduced thickness do not promote a reduction in the wear of the refractory material at the start of the casting process. As would instead be desirable from the start of the process, a correct coupling between plates and rolls would thus prevent a possible penetration of the molten metal between the two contacting walls. Moreover, if such thin coatings that operate under such prohibitive thermomechanical conditions are worn or removed, for instance by molten metal penetration, the problem of the wear of the underlying refractory material crops up anew. An alternative technology is disclosed in JP03207554A and U.S. Pat. No. 5,247,987, wherein side dams are provided with a multi-layer structure in order to promote an initial wearing of the same from the very start of the casting process.
Therefore, it is an object of the present invention to overcome the abovementioned drawbacks by providing plates for side containment in continuous casting machines which are suitable for rapidly adapting to the pattern of the roll edge, thus avoiding cracks along the contact area between the plate and the rollxe2x80x94cracks that can easily lead to molten metal penetrations.
Another object of the present invention is to provide plates for side containment of continuous casting machines which are suitable to overcome the problem of premature abrasion of the same, thereby increasing the duration of the entire casting process and therefore the volume for each individual casting operation.
Therefore, according to the present invention, a refractory plate member is provided for side containment in continuous casting machines of the type including two counterrotating rolls, said plate member including two or more overlapped ceramic and/or refractory layers, wherein the first layer of said plate member is suitable for contacting the rolls and is made of a material that is easily wearable under the action of said rolls during the starting step, and the second layer underlying said first layer made of a material having a very high wear and thermal shock resistance, the plate member being characterized in that said first layer is made of a material including 20 to 30% BN by weight with respect to the total weight; the remaining weight being made of one of more of Al2O3, SiO2, SiC, ZrO2, and/or SiAlON, and by a refractory fibrous casting having an aluminum and/or silicon/aluminum and/or zirconium-based matrix, said casting being realized with Al2O3 and/or SiO2 and/or ZrO2 powders having a size grain of over 60 xcexcm; and said second layer comprises a graphitic carbon material obtained through a hot pressing process starting from a carbon matrix, with a grain size comprised from 30 to 250 xcexcm wherein graphite flakes having a size comprised from 50 to 125 xcexcm and a final porosity from 6 to 15% are incorporated, Advantageously, according to the present invention, the refractory plate for side containment of molten metal in a mould is made of at least one member made of two or more overlapped layers of refractory and/or ceramic material having different abrasion wear resistance characteristics. In fact, the plate member consists of an outer layer made of a material that is quickly wearable by the sliding action of the rolls during the starting of the casting process. Thus, during the starting phase when the plates are pressed against the rolls sides and prior to the pouring of the molten metal, the refractory material is rapidly wearable at the contacting area by the edges of the rolls that indent the plates for a thickness sufficient to ensure optimum containment of the molten metal to be subsequently cast between the rolls.
Advantageously, the coupling obtained with the described step enables avoidance of the formation of cracks in the contact area between the rolls and the side containment plates, and therefore possible molten metal penetrations.
Moreover, according to the present invention, the material forming said outer layer has non-wettability, chemical inertia to the molten metal, and thermal shock resistance, as required by the casting process.
Further, the innermost layer of the plate member, i.e. the layer not directly contacting the molten bath, is advantageously realized in a refractory and/or ceramic material characterized by high hardness, high mechanical high temperature wear, and self-lubricating features.
According to the embodiment of the invention, this allows an optimal side containment of the molten bath, because after the outermost layer is worn, the rolls sides rapidly contact the underlying layer, which is characterized by a very high wear resistance, very small friction coefficient, allowing therefor very limited consumption, and thus enabling the implementation of prolonged castings of thin flat products.
A further advantage provided by the invention is that of maintaining in the central zone of the plate contacting the molten metal, a layer of material having suitable chemical and physical characteristics suitable for resisting molten metal attack, enabling to the underlying layer the sole function of resisting hot wear caused by the casting rolls. In fact, the outer layer is solely removed along the sliding arc of contact area of the rolls.